In the treatment of tumors by ionizing radiation, typically X-rays or gamma rays are used. The ideal in radiation therapy of malignant disease is achieved when the tumor is completely eradicated, and the surrounding normal tissue, in the treated volume, shows little or no evidence of structural or functional injury. The important factor in successful treatment is the difference in radiosensitivity of neoplastic and normal cells. All tissues, normal and neoplastic, are affected by radiation so that radiosensitivity is a relative term. The basic consideration of radiation therapy is that cells that are actively proliferating or that cells which are of a primitive type are more sensitive than normal tissue so that there is usually a considerable margin between doses that are damaging to neoplastic and to normal cells. If this is the case, then a multifraction dose schedule decreases the size of the tumor over time while permitting time between doses for normal tissue to recover. A constant fraction of tumor cells are killed with each treatment, and theoretically the tumor can be completely eliminated with a sufficient number of treatments. However, normal tissue has a memory of its accumulated radiation dose such that a threshold to the total dose acquired over the patient's history is eventually reached. Exceeding this threshold results in unacceptable side effects. Thus, the tumor volume must be reduced sufficiently before the threshold is reached or the cancer is incurable by this modality of therapy.